1. Field of the Invention
The invention relates to a composite membrane useful in electrolytic processes and other chemical separations.
2. Description of Related Art
In electrolytic processes such as the electrolysis of brine or hydrogen chloride, or in the electrowinning of various metals, it is important to provide a separator between anode and cathode compartments. Chemically stable ion exchange membranes made from perfluorosulfonic acid polymer, as described in U.S. Pat. Nos. 3,282,875; 3,718,627; 4,358,545; and 4,329,434, or from perfluorocarboxylic acid polymer, as described in U.S. Pat. Nos. 4,131,740 and 4,734,170, have found broad use as separators. For the electrolysis of brine to produce concentrated caustic, a multilayer membrane involving both perfluorosulfonic acid polymer and perfluorocarboxylic acid polymer has been used as described in Japanese Patent Application Disclosure No. 52/36589.
For reasons of quality, efficiency, cost effectiveness and often safety it is important that the separator be tear-, abrasion-, puncture- and scratch-resistant, yet not so thick or reinforced that its resistance to ionic conduction and concomitant power consumption are excessively high.
Unreinforced perfluoro ion exchange resin membranes are mechanically weak, especially when swollen in aqueous media. For example, characteristic Elmendorf tear strength for a 5 mil, 1100 equivalent weight film is about 80 grams when dry and considerably less when saturated with water. As a consequence, reinforcement mechanisms have been devised in which a fabric, usually made from polytetrafluoroethylene (PTFE) fibers, has been partially or wholly encapsulated by the perfluoro ion exchange polymer. Commercial products reflect this approach. However, it requires about a 5-10 mil thickness of ion exchange membrane to effectively bond to and encapsulate the fabric. Thinner membranes are unsatisfactory since they may not completely cover the fabric on both sides and the integrity of the membrane is impaired. The electrical resistance in aqueous media of this reinforced 5-10 mil structure is considerably higher than that of an unreinforced thinner membrane because of the increased thickness and the reduced effective cross-section available for ion transport because of the encapsulated fabric.
U.S. Pat. No. 4,604,170, in an attempt to address this problem, discloses a composite of a relatively thin continuous perfluoroionomer membrane with a porous, expanded polytetrafluoroethylene (EPTFE) structure rendered hydrophilic by a number of means to provide an electrolytic membrane with low resistance to ionic conduction and moderate tear strength. High levels of tear strength and cut-through and abrasion resistance are not provided by this approach, however. High levels of tear strength and abrasion resistance are not provided by any known prior EPTFE structure which is sufficiently porous to provide acceptably low voltage operation.
U.S. Pat. No. 4,604,170 also claims the use of a multilayer membrane consisting of perfluorosulfonic acid polymer and perfluorocarboxylic acid polymer which is laminated to porous EPTFE for use in brine electrolysis to reduce hydroxyl ion back migration.
Japanese Patent Application Disclosure No. 62-280231 describes a reinforced membrane structure involving a continuous perfluoro ion exchange film laminated to a porous body. The porous body is described as a three-layer laminate involving a fabric or scrim made from a fluorine containing polymer which has been sandwiched and encapsulated between two EPTFE sheets. To provide hydrophilicity and some measure of cohesion, the EPTFE sheets and the fabric are impregnated with a solution of the acidic form of perfluorosulfonic acid polymer. After drying, the porous body sandwich structure is heat laminated with the continuous perfluoro ion exchange sheet. This construction provides an EPTFE interlayer between the fabric and the continuous ion exchange membrane. However, the bond provided by the perfluoroionomer is low, e.g., less than 60 gms/in. peel strength for a 20 strand per inch scrim, and degrades to essentially no strength, e.g., less than 20 gms/in. peel strength, after immersion in hot water for 24 hours.
Japanese Patent Application Disclosure No. 62-280230 describes a composite structure in which a scrim or open fabric is heat laminated and encapsulated between a continuous perfluoro ion exchange membrane and an EPTFE sheet, thus imparting tear strength to the structure. This method of mechanical enhancement is deficient in that relatively thick ion exchange membranes or high equivalent weight membranes must be used to penetrate through the fabric and bond to the EPTFE sheet in order to avoid pinholes and thereby loss of electrolytic integrity. Thicker membranes or high equivalent weight membranes result in higher voltage operation. Also this method is not applicable to composites having more tightly woven fabrics, which in some applications may be desirable to protect the membrane from cutting or abrasive damage.
The present invention overcomes most of the mechanical deficiencies of previous membranes and, surprisingly, provides a membrane having a lower resistance to ionic conduction which approaches that of very thin, unreinforced perfluoro ion exchange polymer membranes.
In the separation of fluids, membranes through which fluids have different permeation rates have been useful in separating mixtures of those fluids. Such films have been wound with macroporous separating meshes which permit free flow of fluids to and from the membrane's surfaces and modules have been constructed. Thin perfluoroionomer films have a very high permeability to water and some other polar molecules, but effective permeation separation modules can not be built from these thin, fragile perfluoroionomer films.
Facilitated transport is a related separation technique wherein a continuous membrane is plasticized or swollen with a liquid. The dissolved liquid complexes with one of the fluids to be separated and selectively facilitates its transport across the membrane. Again, thin perfluoroionomer films offer some unique opportunities for facilitated transport, as, for example, in the separation of amino acids in aqueous media, but the thin perfluoroionomer is not sufficiently strong to undergo module construction or withstand operating pressure differentials. U.S. Pat. No. 4,194,041 provides for a waterproof article which permits the passage of water vapor and includes a hydrophilic layer. Specific claims are drawn to a perfluoroionomer, which permits the passage of water vapor and which is composited with a hydrophobic EPTFE layer which prevents the passage of liquid water. However, the ability to transport aqueous liquids is important not only in electrolytic processes but also in permeation separation and facilitated transport operations.
The composites of the present invention overcome the mechanical strength limitations of thin perfluoroionomer films without significantly reducing the high permeation and transport rates possible with these thin perfluoroionomer films. In addition, the composites of this invention provide for an EPTFE layer which is rendered hydrophilic by coating the interior and exterior surfaces of the EPTFE with a perfluoroionomer without destroying its porosity. In this way, gas locking of the EPTFE structure, which blocks the flow of aqueous liquids to or from the continuous membrane's surface, is avoided.
The structure of the present invention involves the flow and/or mechanical interlocking of the fiber of the fabric into the pores of the EPTFE sheet, resulting in strong bonds, e.g., varying from 80 to 500 gms/in. peel strength, in the absence of an adhesive, which remain essentially unchanged even after long exposure in boiling water. Indeed, in some cases where extensive penetration of the surface layer of the polymer into the sheet is achieved, the EPTFE sheet fails cohesively before the fabric/EPTFE interface can be separated.